Transcript of "Nat"

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NETWORK ADDRESS TRANSLATION (NAT) Providing IP level access between host at a site and the rest of the Internet, using valid globally valid IP address known as NAT. The address used for private network are strongly recommends the following address be used: 10.0.0.0 - 10.255.255.255 172.16.0.0 - 172.31.255.255 192.168.0.0 - 192.168.255.255 These address never appear on the internet. Network address translation (NAT) allows a site to use a set of private addresses for internal communication and a set of global Internet addresses for communication with another site. NAT 2 2/15/2013 2:47:41 AM

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The site must have only one single connection to the global Internet through arouter that runs NAT software. Packet before Packet after translation translation 1 W e 2 NAT b 10.0.0.1 198.1.1.2 SLAN e r v 3 e This software is known as r NAT box, all datagram 4 pass through the NAT box as they travel form the site out to the Internet or form Internet to site. 2/15/2013 2:47:41 AM NAT 3

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NAT Translation Table Creation How the NAT know which Internal host should receive a datagram that arrives form the Internet. So the NAT maintain a Translation Table that is used for mapping. Each Entry of the Table specifies the two item: 1. IP address of a host on the Internet. 2. The Internet IP address of a host at the site. There are several way to initialize the table: 1. Manual Initialization. 2. Outgoing datagrams. 3. Incoming name lookups. 2/15/2013 2:47:41 AM NAT 6

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Continue… 1. Manually Initialization: Configures the translation table manually before any communication occurs. 2. Outgoing datagram: When it receive a datagram for the internal host, NAT create an entry in the translation table as a record of the both host and the destination addressees. 3. Incoming name lookup: It is built as a side-effect of handing domain name lookups. When a host on the Internet looks up the domain name of an internal host to find its IP address, the domain name software creates an entry in the NAT translation table, and then answers the request by sending address G. Thus, from outside the site, it appears that all host names at the site map to address G. Out of this three the mostly implementations of NAT use outgoing datagram to initialize the table. 2/15/2013 2:47:41 AM NAT 7

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Multi Address NAT As we have simplistic the NAT perform a 1-to-1 address mapping between external address and internal address. But NAT want to permits concurrency by retaining the 1-to-1 mapping, by allowing the NAT box to hold multiple Internet address. This is Known as Multi-Address NAT which is done by the NAT box. It set of K globally valid address G1,G2,…,Gk. Thus, multi-address NAT allow up to K internal host to access a given destination concurrently. 2/15/2013 2:47:41 AM NAT 9

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Continue… The table contain the four internal computers that are currently accessing destination on global Internet and all of them are using TCP. In the table two internal host accessing protocol port 80 (web server) on computer 128.10.19.20. However the source port cannot be guaranteed it turn the two internal host have same port number. To avoid this NAT assign a unique port number to each communication that is used on the Internet. 2/15/2013 2:47:41 AM NAT 11

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Continue… As in the table the first two item correspond to the TCP connection. Such as: 10.0.0.5, 21023, 128.10.20, 80 10.0.0.1, 386, 128.10.20, 80 After the computer in the Internet that receive datagram after the NAPT perform the translation such as: G, 14003, 128.10.20, 80 G, 14010, 128.10.20, 80 Where G is the Globally valid address of the NAT box. 2/15/2013 2:47:41 AM NAT 12

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Pros and Cons Of the NAT Pros: Cons: NAT increases the design IP addresses and ports lose space available for an their end-to-end significance. intranet. Increases amount of state Saves address space. information in the network. Can be deployed incrementally and (almost) Increases the complexity of transparently. an edge router (protocol dependent). Method of choice for home network. Some protocols don‘t work across NAT’s. 2/15/2013 2:47:41 AM NAT 13